1. Field of the Invention
This invention relates to A.C. power supplies particularly for use with single-phase traction supply systems.
2. Description of Related Art
Single phase A.C. traction systems are well established, especially at 25 kV. This relatively low voltage is derived from a high voltage system by transformation at a substation. Current is drawn from a main supply (catenary) conductor at 25 kV and returns, at earth potential, through the track and a conductor bonded to the track at intervals and together constituting an earth return conductor. The 25 kV system impedance is relatively high: thus voltage regulation is a problem and it is normal to design for locomotive operation in the range +10-30% i.e. 27.5 to 17.5 kV. Even so, distances between feeder stations must be relatively short, especially for heavily loaded conditions (e.g. mineral trains). In such cases a 50 kV supply system has sometimes been found advantageous to reduce supply voltage drops while still using 25 kV locomotives. The traction system catenary wire is fed from one terminal of the 50 kV supply and a supplementary feeder wire is supplied from the other terminal. 50/25 kV autotransformers are connected at intervals between the catenary and the feeder wire with their tapping point earthed and connected to the traction return conductor. Thus both the catenary wire and the feeder will operate at 25 kV from earth, but with 180.degree. phase displacement.
There are several other features of traction supply systems which may require alleviating measures. The locomotives do not draw sinusoidal currents and commutation effects of the on-board converters excite the natural resonances of the traction system. Harmonic filters and/or damping circuits are sometimes necessary to prevent excessive levels of harmonic currents flowing into the supply system and to prevent the voltage distortion on the 25 kV traction system itself from reaching levels which would cause damage to other equipment, including the locomotives themselves.
The single phase traction loads cause negative phase sequence currents to flow in the HV supply system. Unless this HV system is strong (i.e. has low impedance), the consequential negative phase sequence voltages may reach levels which would be damaging to other equipment, particularly machines fed from the HV system. In order to reduce the levels of unbalance, the traction supplies at a given traction substation are sometimes arranged to be supplied from different phases of the HV supply, but emergency operating conditions normally entail that the total maximum load may still be supplied from one phase only. Where the negative phase sequence voltages on the HV system are intolerable, high-speed controllable phase-balancing devices may be connected to the HV system to limit the negative phase sequence voltage.
FIG. 1 of the accompanying drawings shows a well-known basic arrangement often adopted for converting a single-phase load of any power factor into a balanced 3-phase load of unity power factor using only reactive elements for the balancing equipment.
A single-phase load 11 in phase A-B, has a load rating represented by P-jQ is inductive, (absorbing reactive power), or P+jQ if capacitive, (generating reactive power). In parallel with the single-phase load 11 in phase A-B is connected a device 13 to give reactive power generation, i.e. capacitance power rating +Q, sufficient to balance the lagging reactive power, Q, of the load (or, if the load power factor is leading, a device 15 shown in broken lines to give reactive power absorption, i.e. inductance power rating -Q, sufficient to balance the leading reactive power of the load). The single-phase load is thus converted to unity power factor and of value P. Into phases B-C and C-A are connected other reactive devices, 19 of value +jP/.sqroot.3 and 17 of value -jP/.sqroot.3.
The compensating load in phase B-C must be capacitive and in phase C-A must be inductive for a system in which the positive phase sequence is A-B-C.
If the load in phase A-B is a variable load, then the phase-balancing circuit components 13/15, 17 and 19 in the three phases must also be varied in a pro-rata manner to P and Q as appropriate in order to maintain the load seen by the supply system as a unity power factor load, albeit variable in magnitude.
Traction loads are not constant. They vary due to the effects of gradients on the track, starting, stopping, shunting and the response of the driver to signals etc. In addition, power is instantaneously shut off and rapidly restored as locomotives enter and leave the neutral sections which are used to isolate the different sections of the traction supply system. Such changes will cause corresponding voltage fluctuations on the HV system, which may in turn cause disturbance or annoyance to other consumers and may therefore require to be attenuated to a tolerable level.
In some cases the power factor of the traction load is poor and there would be merit in the application of power factor correction equipment.
Although the various techniques for counteracting these adverse effects of traction systems are well-known and established, their independent application generally involves some lack of economy of materials due to secondary effects. Thus in a case where both traction harmonics and unbalance effects are unacceptable to the HV system, the connection of single-phase harmonic filters to the traction supply system would impose an additional fixed negative phase sequence load which must then itself be compensated. Any excessive harmonics produced by the phase-balancing device will normally need to be separately absorbed by additional filters; the phase-balancing device will usually be supplied via a stepdown transformer from the HV system, in which case the additional filters will normally be connected to its LV side. Although it may seem advantageous to use a common set of harmonic filters connected directly to the HV system to absorb both the traction harmonics and the phase-balancer's harmonics, no advantage is usually realised because, in order to be effective, the filters must have a much higher admittance (and therefore higher fundamental rating) when connected to an HV system compared with an LV system (because of the lower magnitude of HV sytem harmonic impedance). In addition, the transferral of capacitive MVAr from LV to HV side of the phase-balancer will usually increase the rating of the balancer's stepdown transformers and, for the traction system, the HV filters will no longer be properly effective in reducing the harmonic voltage distortion on the catenary system itself.